WE DESIRE TO INSULATE A CHURCH. IT HAS POST AND BEAM FRAMING WITH 2” – 10” PURLINS ON TOP OF THE RAFTERS SUPPORTING THE SHEATING AND CEILING UNDERNEATH. AT THE PRESENT TIME THERE IS NO VENTILATION BECAUSE OF THE ROOF DESIGN THERE SEEMS TO BE NO MOISTURE PROBLEM. MY QUESTION IS, IF WE BLOW IN INSULATION AN FILL THE PURLINS WILL WE HAVE A MOISTURE PROBLEM DUE TO NO AIR CIRCULATION? AT THE PRESENT TIME THERE IS A SMALL AMOUNT OF INSULATION IN THE CAVITIES.
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What kind of pitch on the ceiling?
If you insulate you will have condensation on the surface uninsulated near the roof. One needs to ventilate the bottom surface or find a new roof after the roofing material rots out if the soggy insulation doesn't rain down first.
This isn't as easy as putting on a blanket with the blown in insulation. You need a vapor barrier twords the interior, the "blanket" and venting above the insulation. I suspect there are more materials involved.
Where is the building? In the south or the north? do you use A/C or heat most of the time.
Too many questions with your posting. Describe in detail from the roofing material down to the open space please.
THE ROOF IS ABOUT A 6/12. THE INSULATION THAT EXIST ON THE BOTTOM OF THE PURLINS AT THIS POINT HAS BLACK PAPER ATTACHED TO WHAT I BELIEVE IS FIBERGLASS INSULATION ABOUT 3 1/2'' THICK. BELOW THAT THERE IS 3/4 NAILERS THAT RUNS VERTICALLY ON WHICH COMPOSITE BOARDS OR NAILED. SO THAT AIR SPACE EXIST ON THE INTERIOR SIDE. THE CHAPEL IS LOCATED IN WISCONSIN HEATED BUT NO AIR CONDITIONING. FROM THE FLOOR TO THE RIDGE IS ABOUT 20' TO THE SIDEWALLS ABOUT 10'. MY THOUGHT WAS TO BLOW IN INSULATION AND FILL THE CAVITIES COMPLETLY WHEN WE TEAR OFF THE OLD SHINGLES.
Here is the drill.
Breathing people let out moisture as well as the local humid air. When they do that there is a phenomenon that the moisture condenses on the least warm location. (think bathroom mirror) That point is the location where the roofing is in your church.
The moisture collects on the cool surface and if it is porous (plywood) it gets absorbed into the material. Over time it will mold and deteriorate. UNLESS IT IS VENTILATED.
To your good luck you have minimal insulation and the normal cycle of warm weather and heating systems has probably dried out the dewpoint surface. (check your roof from the outside and see if it sags between the rafters. If so you should replace the roof sheeting as well as it has already deteriorated) Hopefully this is not the case.
So. That is the problem that ventilation tries to cure or at least abate.
The black paper is tarpaper? or is it something else? It sounds like the original builder was expecting God to come sooner rather than later as the insulation is pretty substandard as is the vapor barrier. In fact the air gap around the black paper could be a fire hazard.
Vapor barrier is supposed to be in contact with the ceiling surface and the insulation above that. The barrier needs to be in contact so that it is not a flammable tunnel for flames. Above the vapor barrier should be your insulation.
On a 6/12 pitch you are going to have a lot of insulation falling down the slope if you use blown in. Batts or foam board would be better choice of insulating material in my estimation. The existing insulation is not worth much so if you threw it away you will probably save more time than if you tried to Band-Aid it into use.
I guess another approach would be to get the foam trough that is found at Home Depot / Menards / etc. and use that as a way to keep air at the roof boards while giving you a location to sneak insulation underneath. However you sound as if you'd have to poke that stuff 15 foot up the ceiling line unless you open up the ceiling in the middle of the run.
In any case you need a ridge vent and soffit vents in order to get the flow thru of fresh air to wick away the moisture your crowd creates.
How long did the current roof last? 25 to 30 years would be normal to good. if it is far less you may be experiencing the decay from the moisture that is causing leaks.
Another thought might be to put a coating of ceiling tile on the surface. It would be attractive and provide a couple points of R value.
Without seeing it I'd say the above suggestions are just that.
Consider losing the CAPS next time you write. It is computer etiquette not to use those. Capitals are reserved for shouting ;-)
Hopefully someone else will chime in here.
Thanks for all your help